The present invention relates to a process control instrument. In particular, it relates to a pressure transmitter with an isolator diaphragm.
Transmitters which sense pressure have a pressure sensor coupled to at least one isolation diaphragm. The transmitters typically attach to a flange and include an opening which is aligned to receive process fluid from a passageway in the flange. The isolation diaphragm is positioned in the opening, and isolates the pressure sensor from corrosive process fluids being sensed. Pressure is transferred from the isolation diaphragm to the sensor having a sensing diaphragm through a substantially incompressible isolation fluid carried in a passageway. U.S. Pat. Nos. 4,833,922 entitled MODULAR PRESSURE TRANSMITTER and 5,094,109 entitled PRESSURE TRANSMITTER WITH STRESS ISOLATION DEPRESSION show pressure transmitters of this type.
Pressure transmitters frequently include a weld ring positioned within the opening surrounding the isolation diaphragm. The weld ring can be welded to the transmitter body and to the isolation diaphragm, thus securing the isolation diaphragm to the transmitter body. The weld ring can also be used to support a seal, such as an O-ring. When the transmitter is attached to the process flange by an attachment force, such as provided by bolts, the seal and weld ring are compressed against the flange to prevent process fluid from leaking past the opening.
The attachment forces which compress the weld ring against the flange tend to deform or deflect portions of the weld ring. Frequently, the resulting deformation of the weld ring causes the attachment forces to be transferred to a concentrated portion of the isolation diaphragm. This in turn can damage the isolation diaphragm. Even if the isolation diaphragm is not significantly damaged by the concentrated forces transferred by the weld ring, the forces can dent or crease the diaphragm, requiring that the transmitter be recalibrated.